From the Wizard101 website:

Beastmoon Monster Mayhem Event!

"A little mischief and mayhem never hurt anyone except the bad guys! Now through Monday, September 26th, get ready to embrace your inner beast and team up with allies in an attempt to best multiple waves of monsters! Will you come out on top? See Event "

See the Announcement here:

See additional information here: Basic:Beastmoon Monster Mayhem

And see this thread from @Pegasus Unbound: Beastmoon Monster Mayhem "Simplified".